The present invention relates to a printing apparatus which can efficiently print both large and small numbers of copies in two different printing modes.
The so-called "digital printing machine" is currently popular as this printing machine combines high printing speed with low operating cost. In the digital printing machine, a heat-sensitive stencil sheet comprising a thermoplastic film is molten and perforated by a heating means, such as a thermal head which emits heat as a pattern of dots in accordance with the desired image information including letters, figures and photographs, having previously been converted to electrical signals. Thereafter, the stencil sheet is wound around a printing drum containing stencil printing ink therein to transfer the printing ink through perforations of the stencil sheet onto printing paper.
However, when the stencil sheet is perforated in the digital printing machine, problems arise, including deficient perforations, failure of conveyance of stencil sheets and wrinkling of the stencil sheet due to exertion of uneven pressure upon pressing the stencil sheet to the thermal head.
Furthermore, while the conventional digital printing machine is useful for printing a large number of copies, the machine is expensive when used to generate a small number of copies.
Under the circumstances, a possibly remedy to the problem would be a printing machine equipped with heat-sensitive recording paper or thermal transfer recording paper for printing a small number of copies. However, these modifications would require a larger printing machine, and would complicate the operation of the printing machine, since both printing paper and recording paper must be ready for use in the modified printing machine.
An alternative printing machine has been proposed, in which one kind of regular paper can be printed by an electrophotographic process when a small number of copies are made, and by a stencil priming process when a large number of copies are needed. However, the printing system as a whole would be costly and unduly large in size.
Furthermore, when color printing is effected by the digital printing machine, a plurality of printing drums each containing stencil printing ink of a different color is required. The plurality of drums may be interchanged with one another in the printing machine when printing is effected in different colors on different regions of paper. However, implementation of the interchange is difficult, thereby reducing operating efficiency.